Conventional articles of athletic footwear have included two primary elements, namely an upper member and a sole structure. The upper member provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper member may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure generally is secured to a lower portion of the upper member and generally is positioned between the foot and the ground. In addition to attenuating ground or other contact surface reaction forces, the sole structure may provide traction and control foot motions, such as pronation. Accordingly, the upper member and sole structure operate cooperatively to provide a comfortable structure that is suited for a variety of ambulatory activities, such as walking and running.
The sole structure of athletic footwear generally exhibits a layered configuration that includes a comfort-enhancing insole, a resilient midsole, and a ground-contacting outsole that provides both abrasion-resistance and traction. The midsole is typically the primary sole structure element that attenuates ground reaction forces and controls foot motions. Conventionally, the midsole was formed from an open- or closed-cell polymer foam material, such as ethylvinylacetate or polyurethane, that resiliently compresses under an applied load to attenuate ground reaction forces.
Impact-attenuating elements for footwear have been developed for various specific applications. For example, for athletic footwear, which may be subjected to relatively high impact forces, e.g., from running, jumping, twisting, changing directions, and the like, it may be desirable to provide increased impact-attenuation capabilities and/or increased spring-back or recovery capabilities. Thus, in addition to polymer foam materials, midsoles may also include, for example, one or more fluid-filled bladders and moderators. Other examples of such impact-attenuating elements have been described in U.S. Patent Application Publication No. 2006/0065499, titled “Impact Attenuating and Spring Elements and Products Containing Such Elements,” filed Sep. 27, 2004; U.S. Patent Application Publication No. 2006/0064900, titled “Impact Attenuating Devices and Products Containing Such Devices,” filed Sep. 27, 2004; and U.S. Patent Application Publication No. 2007/0119074 titled “Impact-Attenuation Members and Products Containing Such Members,” filed Jun. 5, 2006, each of which application is incorporated herein by reference in its entirety.
These known impact-attenuating elements are symmetric from front-to-back and side-to-side, and thus, they provide symmetric impact-attenuation capabilities from one end to the other and from one side to the other. In other words, the stiffness and deflection characteristics of any of these known individual impact-attenuating elements are the same at opposite ends or opposite sides. Due to their symmetric nature, when subjected to an external, centrally-applied impact load or an evenly distributed load, they deflect or react evenly to the loading.
Ideally, the stiffnesses of different portions of an article of footwear will be tailored to the footwear application. For example, a certain midsole stiffness may be desired in the heel zone for heel strike during running, while another midsole stiffness may be more appropriate for walking impacts. Thus, a stiffer impact-attenuating element may be ideal for medial heel posting, while a softer stiffness may be desired for the lateral heel area. Generally, the perimeter sidewall impact-attenuation and stability desired in a midsole is different from the interior impact-attenuation requirements.
It would be desirable to provide an impact-attenuation member that reduces or overcomes some or all of the difficulties inherent in prior known devices. Particular objects and advantages will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain embodiments.